Signal Transducer and Activator of Transcription 3 (STAT3) is an oncogenic transcription factor (TF) and a central immune checkpoint regulator. Tumorigenic and tolerogenic roles of STAT3 in acute myeloid leukemia (AML) and other human cancers, as well as in the tumor-associated immune cells are well-defined and provide solid scientific premises for therapeutic STAT3 inhibition. To overcome the challenge imposed by lack of pharmacological inhibitors of STAT3, we recently developed a strategy for myeloid cell-selective STAT3 inhibition in vivo. Tethering to a synthetic Toll-like Receptor 9 (TLR9) agonists, CpG ODNs, allowed for targeted delivery of a STAT3 decoy oligodeoxynucleotide (STAT3dODN) into myeloid cells, such as AML cells and tumor-associated immune cells. Our preliminary studies demonstrated that intravenous injections of CpG-STAT3dODN lead to regression of disseminated human and mouse acute myeloid leukemia (AML) in murine models. In immunocompetent mice, STAT3-blocking/TLR9-stimulation triggered differentiation of leukemic cells to antigen-presenting cell (APC) phenotype rather than direct cytotoxicity. The immunogenicity of AML-APCs induced systemic CD8/CD4 T cell-mediated immunity and eliminated disseminated leukemia, including leukemic stem/progenitor cells, with no detectable toxicities to normal immune/hematopoietic stem cells. We propose to elucidate molecular/cellular mechanisms of CpG- STAT3dODN-induced AML immunogenicity and to characterize role of T cell-mediated immunity in AML rejection. Better understanding of the CpG-STAT3dODN effect on leukemia cell and T cell compartments will allow for the design of optimal combination of TLR9-targeted STAT3 inhibition with T cell-based therapies which will be validated within this proposal. These studies will accelerate development of novel, effective and safe nucleotide-based immunotherapeutic strategies for cell-selective targeting of STAT3 in AML and potentially other hematologic malignancies.